1. Field of the Invention
The present invention relates to a controller for an electric power steering system which assists a steering operation by utilizing a driving force generated by an electric motor.
2. Description of Related Art
Electric power steering systems are conventionally utilized which employ an electric motor as a source of a steering assist force to be applied to a steering mechanism. A controller for such an electric power steering system controls the electric motor on the basis of a vehicle speed and a steering torque applied to a steering wheel. More specifically, detection signals from a torque sensor for detecting the steering torque and a vehicle speed sensor for detecting the vehicle speed are inputted to the controller. The controller determines an electric current command value in accordance with the detection signals inputted from the torque sensor and the vehicle speed sensor, and feedback-controls the electric motor on the basis of the electric current command value.
The electric power steering system of this type suffers from a response delay (a delay in generation of the steering assist force in response to a change in steering torque) due to the inertia of the electric motor when the steering wheel is dynamically turned in opposite directions in a slalom steering operation or when the steering wheel is quickly turned to prevent the motor vehicle from diverting from a lane during traveling on a highway. With the response delay, a driver experiences a heavier steering feeling and an entrapped feeling when operating the steering wheel.
It is an object of the present invention to provide an electric power steering controller which is capable of reducing a response delay which may occur due to the inertia of an electric motor.
The electric power steering controller according to the present invention comprises: a basic assist electric current generating section for generating a basic assist electric current value in accordance with a steering torque; a first inertia compensation value generating section for calculating a first steering torque differential value equivalent corresponding to a time-based differential value of the steering torque and for generating a first inertia compensation value in accordance with the first steering torque differential value equivalent; a target electric current value generating section for generating an assist target electric current value by adding the first inertia compensation value generated by the first inertia compensation value generating section to the basic assist electric current value generated by the basic assist electric current generating section; and a motor driving section for driving an electric motor on the basis of the assist target electric current value generated by the target electric current value generating section.
The controller preferably further comprises a second inertia compensation value generating section for calculating a second steering torque differential value equivalent corresponding to a time-based differential value of a steering torque component obtained by removing a high frequency component from the steering torque and for generating a second inertia compensation value in accordance with the second steering torque differential value. In this case, the target electric current value generating section is preferably adapted to generate the assist target electric current value by adding the first inertia compensation value and the second inertia compensation value to the basic assist electric current value.
In accordance with the present invention, the first inertia compensation value is generated in accordance with the first steering torque differential value equivalent corresponding to the time-based differential value of the steering torque including the high frequency component, and the assist target electric current value is generated on the basis of the first inertia compensation value thus generated. The assist target electric current value generated on the basis of the first inertia compensation value is a value corrected for improvement of the responsiveness when the steering torque varies with a high frequency. Therefore, the electric motor controlled on the basis of the assist target electric current value can generate a steering assist force in quick response to a steering operation in which an operation member is quickly and slightly turned.
The second inertia compensation value is generated in accordance with the second steering torque differential value equivalent corresponding to the time-based differential value of the steering torque component obtained by removing the high frequency component from the steering torque. Therefore, the assist target electric current value including the second inertia compensation value is a value corrected for improvement of the responsiveness when the steering torque is gradually changed. Therefore, the electric motor controlled on the basis of the assist target electric current value determined in consideration of the second inertia compensation value can generate the steering assist force in quick response to a slalom steering operation in which the operation member is dynamically turned.
This eliminates the possibility that the driver experiences a heavy steering feeling or an entrapped feeling when operating the operation member.
The second inertia compensation value generating section may comprise a low pass filter for removing the high frequency component from the steering torque, and may be adapted to generate the second inertia compensation value in accordance with a time-based differential value of a steering torque component obtained by filtering the steering torque through the low pass filter.
The low pass filter may be implemented on a hardware basis, or through a computation for the removal of the high frequency component.
The foregoing and other objects, features and effects of the present invention will become more apparent from the following description of the preferred embodiments with reference to the attached drawings.